The present invention relates to a structure of a dewar for SQUID which keeps a SQUID magnetometer at a cryogenic temperature.
In recent years, a high sensitive magnetism measurement apparatus using a SQUID (Superconducting Quantum Interference Device) is used for measuring a very weak magnetic field produced from a living body or the like. The SQUID magnetism measurement apparatus can measure a very weak biomagnetic field produced from a brain or a heart, and position of a current source producing the magnetic field can be estimated from the distribution of the biomagnetic field. Therefore, the magnetism measurement apparatus has attracted considerable attention as a medical equipment for diagnosing and determining a function of brain or a function of heart.
The SQUID is a magnetic-electric converting element operated under a superconducting state, and a magnetic sensor using the SQUID is cooled down to a cryogenic temperature using liquid helium or the like in a dewar. A conventional dewar for SQUID is disclosed in Japanese Patent Application Laid-Open No. 11-233839.
In a case where a very weak magnetic field is measured using the SQUID magnetometer, if an eddy current is generated in an inner container for holding the SQUID magnetometer and the liquid helium or an outer container for forming a vacuum enclosure between the inner container and the outer container due to the measured magnetic field or an environment magnetic field, the eddy current ill affects the measured magnetic field. Therefore, the inner container and the outer container of the dewar for SQUID is made of a non-magnetic and non-conductive material, for example, made of FRP (glass-fiber reinforced epoxy resin), and assembled by being adhered with a polymer.
In a conventional dewar made of stainless steel used for cooling a superconducting magnet or the like, helium gas does not penetrate through the wall of the dewar, and leakage of helium gas can be prevented through a joint by welding the joined portion. Therefore, the helium penetration itself has not been treated as an important issue. However, in the FRP dewar, helium gas penetrates through the FRP wall due to the material property of FRP. That is, the helium gas in the inner container penetrates through the wall of the inner container to be discharged into the vacuum enclosure. Since the helium gas discharged into the vacuum enclosure (leak helium gas) reduces the degree of vacuum of the vacuum enclosure, the thermal insulating performance of the vacuum enclosure is deteriorated. That is, evaporation of the liquid helium in the inner container causes to deteriorate the thermal insulating performance, the amount of heat transfer into the liquid helium from the external is increased, and the evaporating amount of the liquid helium is further increased. Therefore, the liquid helium evaporates in a short time, and the SQUID magnetometer is exposed above the liquid surface of the helium to be broken down, which makes the magnetism measurement impossible.
In conventional technology, in order to solve the above problem, the decrease of vacuum degree in the vacuum enclosure is prevented by arranging an absorbent (activated charcoal or the like) capable of absorbing helium gas at a temperature below 20 K in the vacuum enclosure in the outer periphery of the inner container of the dewar to absorb the leak helium gas.
When the magnetism measurement apparatus is being operated for a long time period, the liquid helium evaporates to lower the surface level of the liquid helium. In such a case, the wall surface temperature of the inner container above the level of the liquid helium surface in the inner container is increased above 20 K due to heat entering from parts of the room temperature, and the temperature of the absorbent arranged in the corresponding portion is also increased above 20 K. Therefore, the temperature of the absorbent arranged in the portion above the level of the liquid helium surface exceeds the limit temperature capable of absorbing helium gas, and the leak helium gas absorbed to the absorbent is again discharged into the vacuum enclosure. On the other hand, since the wall surface of the inner container below the level of the liquid helium surface in the inner container is continued to be cooled at a temperature below 20 K by the liquid helium, the absorbent arranged in the corresponding position absorbs the leak helium gas discharged from the absorbent arranged above the level of the liquid helium surface. That is, the absorbent arranged below the level of the liquid helium surface re-absorbs the helium gas discharged from the absorbent arranged above the level of the liquid helium surface, and as the result the degree of vacuum of the vacuum enclosure is maintained.
However, since the liquid helium evaporates in a short time, liquid helium needs to be supplied every time interval. That is, since the process of lowering the surface level of the liquid helium described above (the process that the absorbent in the lower portion absorbs the leak helium gas which the absorbent in the upper portion has absorbed) repetitively occurs every cycle of supplying liquid helium, the absorbent in the lower portion reaches the limit of helium absorption power to be brought in the saturation condition. In that state, when liquid helium is supplied so that the liquid surface of the liquid helium reaches a position of the unsaturated absorbent in the upper portion, the absorbent in the upper portion is cooled down to a temperature lower than 20 K to absorb the leak helium, and accordingly the pressure in the vacuum enclosure is decreased to recover the thermal insulating performance and the evaporating amount of the liquid helium is returned to the original state. However, when the liquid surface of the liquid helium is brought to a level below the top end position of the saturated absorbent region, the degree of vacuum is decreased to increase the evaporating amount of the liquid helium. That is, the evaporating rate of the liquid helium is kept to the initial value when the liquid surface of the liquid helium is above the top end position of the saturated absorbent region, but the evaporating rate of the liquid helium is increased to rapidly consume the liquid helium when the liquid surface of the liquid helium is below the top end position of the saturated absorbent region.
Further, by repeating the process described above, the top end position of the saturated absorbent region is further raised, and consequently the evaporating period of liquid helium is further shortened. As the evaporating period of liquid helium is shortened and number of liquid helium supply times is increased, number of occasions of causing expensive liquid helium to evaporate is further increased due to entering of heat at liquid helium supplying work.
An object of the present invention is to provide a dewar for SQUID in which an amount of leak gas discharged from absorbent arranged in the upper portion higher than the liquid surface level of liquid helium is small even if the liquid surface level of the liquid helium is lowered, and accordingly the cycle of supplying liquid helium can be kept long for a long time, and to provide a biological magnetism measurement apparatus using the dewar for SQUID.
In order to attain the above object, a dewar in accordance with the present invention comprises an inner container for holding a SQUID magnetometer and a coolant, the inner container being made of a non-magnetic and electrically non-conductive material; an outer container for forming a thermal insulating space between the inner container and the outer container, the outer container being made of a non-magnetic and electrically non-conductive material; and a gas absorbing means provided inside the thermal insulating space, and the dewar further comprises a non-magnetic thermal conducting means in contact both with the gas absorbing means arranged at a level higher than a level of a holding position of the SQUID magnetometer and with a position of the inner container at a level lower than a level of the gas absorbing means.
By the structure described above, since the thermal conducting means is in contact with a position of the inner container at a level lower than the gas absorbing means even if the coolant held in the inner container evaporates to lower the liquid surface of the coolant in the inner container, the gas absorbing means can be maintained at a temperature of the coolant held in the inner container by the thermal conducting means. Accordingly, the gas absorbing power of the gas absorbing means can be maintained.